1. Field of the Invention
The present invention is aimed at proposing an improvement in systems of shielding against the electromagnetic disturbances generated by an inductor used in induction cookers.
In an induction cooker, a cooking heater is conventionally formed by an inductor comprising one or more coils which may or may not be concentric, the inductor being powered by a generator that delivers AC current at a frequency of about 20 to 50 KHz. The inductor through which this current flows produces currents, by induction, in the walls and at the bottom of a container placed on top of this inductor. These currents heat the container and then the substance to be cooked in the container.
It is known that an inductor of this kind is liable to generate parasitic electromagnetic fields, also called disturbances, that are radiated towards the exterior of the cooker. Such a situation may occur, for example, when the container to be heated has a diameter smaller than the size of the inductor.
Apart from the unnecessary loss of electromagnetic fields produced by the inductor, these parasitic fields give rise to disturbances in the working of radioelectrical instruments in the vicinity of the cooker. They must therefore be reduced to the minimum.
Furthermore, by virtue of its construction, an inductor induces not only an electromagnetic field towards the container to be heated but also a rear magnetic field, namely a field beneath the inductor. This raises the risk of jeopardizing the efficient working of the electric components of the apparatus and of giving rise to disturbances by conduction hereinafter called conduction disturbances.
2. Description of the Prior Art
Many techniques have already been proposed to reduce radiated disturbances and/or conduction disturbances.
For example, there is a known way of reducing the outward parasitic electromagnetic field to the minimum by accompanying the coil of the inductor with a counter-turn, for example a counter-turn of a compensation coil, wound around the heating coil so that the current flowing through this coil is opposite in its direction to the current flowing through the heating coil. The results obtained by this method are however not optimal. Furthermore, this method is ill-suited to a case where the inductor is formed by several heating coils in a concentric arrangement. Indeed, if a single counter-turn is placed around the external coil, the parasitic electromagnetic fields created by the internal coils are not compensated for. However, if a counter-turn is placed between each heating coil, there is a risk that it might disturb the working of the cooker since it is covered by a container (giving rise to a drop in the reactance of the coil, heating by eddy currents in the counter-turns that are difficult to dissipate., cold zones on the walls of the container due to the local cancellation of the magnetic field, etc.).
Furthermore, it is also known that the rear parasitic radiation of the inductor can be avoided by using a ferrite-based magnetic circuit located beneath the inductor. A magnetic circuit such as this however has limitations during operation because it may, under certain conditions, lose its magnetic properties, especially if the temperature of the ferrites used reaches the Curie temperature corresponding to these ferrites.
Another known approach, described for example in the document FR 2 649 576, consists in surrounding the inductor with a shielding cage made of a non-magnetic material, the bottom of the cage being also closed by means of a plate made of non-magnetic material. This approach provides for a reduction in the radiated disturbances and conduction disturbances defined here above. However, it is better suited to professional cookers and can be hardly transposed to the field of large-scale consumer applications wherein it is necessary to design cookers that are as compact as possible.
Now, in the document FR 2 649 576, the bottom of the shielding cage must be at a minimum distance from the inductor and this minimum distance is too great for large-scale consumer applications.
Furthermore, the different plates used to form the shielding cage are the site of eddy currents and, in turn, generate conduction disturbances and/or radiated disturbances.